In a puzzling experiment, the researchers found that astronauts who ate more salt retained more water.

Researchers from the German Aerospace Center (DLR), the Max Delbrück Center for Molecular Medicine (MDC), and Vanderbilt University examined the connection between salt intake and drinking using a mock flight to Mars.

Scientists have known that increasing a person’s salt intake stimulates the production of more urine – it has simply been assumed that the extra fluid comes from drinking.

They simulated a long space voyage using an environment in which every aspect of a person’s nutrition, water consumption, and salt intake could be controlled and measured.

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The studies were carried out by Dr Natalia Rakova from the Max Delbrück Center for Molecular Medicine and colleagues, and used two groups of ten male volunteers sealed into a mock spaceship for two simulated flights to Mars.

A salty diet caused the subjects to drink less, triggering a mechanism to conserve water in the kidneys.

The project revises scientists’ view of the function of urea in our bodies.

Prof. Friedrich C. Luft, Managing Director of the Charité University Hospital, Berlin, said: ‘It’s not solely a waste product, as has been assumed.